Anisotropic Properties of a Mid-Lithospheric Discontinuity in eastern North America

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Abstract

During the last few years the occurrence of a mid-lithospheric discontinuity
(MLD) has been observed in lithospheric studies of cratons. Studies using
receiver functions have focused mainly on extracting high resolution information
about the depth and extent of this discontinuity. Surface wave studies
detect a change in anisotropy occurring over the same interval where the
MLD is mapped, but no one has yet investigated whether these two features
actually coincide. The nature of the MLD could therefore be anisotropic. In
this thesis I use an new database of earthquake recordings to investigate the
MLD in eastern North America. The focus is primarily on station HRV near
Boston, containing 486 receiver functions with good backazimuthal coverage.
Through forward modeling of receiver functions and harmonic decomposition,
I find that the best fit to the MLD at HRV is obtained without a
velocity reduction, but with a change in anisotropy. A change from a horizontal
axis of anisotropy to a plunging axis provides the simplest model
to reproduce the observed HRV data. I attribute this change of anisotropy
to a division between ”frozen-in” anisotropy from the formation of the Appalachian
chain in the upper lithosphere, and reworked fabric in the lower
lithosphere from heating by the mantle plume of the Great Meteor hotspot.
This interpretation implies that the fabric in the lower lithosphere is a localized
phenomenon, which should be distinguished from the craton-wide MLD
signal observed in North America. Regional profiles indicate a reduction
of MLD signal strength towards the craton, suggesting that the lower lithospheric
layer mapped in previous craton wide studies does not extend outside
the craton boundaries. However, such an interpretation cannot be based on
only one station, and should be tested with an expanded study of the entire
eastern North America using semi-automated inversion methods.